Jimmy Roberts is a socially awkward child that was hit by a train during a seventh grade field trip to Gollyworld Theme Park. This caused severe injury to his brain and he was in desperate need for a brain transplant so they decided to give him the brain of Milt Appleday, the deceased founder of Gollyworld. Due to having Milt's brain, he was able to see cartoon characters just like he could. He serves as the titular deuteragonist of Out of Jimmy's Head and Re-Animated.
The reason he's able to retain his old memory and personality is because the surgeons managed to mantain his personality gland.